By finding out how pure and large the acceptor domain is and what the interface of the donor domain looks like, efficiency can be increased

Berkeley Lab scientists have measured various aspects of an organic solar cell for the first time, successfully increasing efficiency with this knowledge.

The study, which was conducted by an international team of scientists at Berkeley Lab and led by Harald Ade, is the first to successfully map the size and purity of domains in organic solar cells to increase their performance.

For photovoltaic devices to compete with fossil fuels for energy collection, they must convert sunlight to energy at a very efficient level. Solar cell conversion depends on excitons, which are electron pairs that are energized by sunlight. These excitons must get to the interface of the donor and acceptor domains as quickly as possible to avoid losing energy as heat.

The efficiency of polymer-based organic photovoltaic cells depends on the purity of the donor and acceptor domains. So by finding out how pure and large the acceptor domain is and what the interface of the donor domain looks like, efficiency can be increased.

"The combination of these three ALS beamlines enabled us to obtain comprehensive pictures of polymer-based organic photovoltaic film morphology from the nano- to the meso-scales," said Brian Collins, the paper's co-author. "Until now, this information has been unattainable."

Using the ALS beamlines, the team studied the polymer/fullerence blend PTB7:PC71BM in thin films made from chlorobenzene solution. It did this with and without the addition of the solvent diiodooctane, and the films were composed of droplets in where the dominant acceptor domain size without the additive was about 177 nanometers.

Adding the solvent reduced the acceptor domain size down to about 34 nanometers while maintaining the film's composition and crystallinity.